Zone Issue: Upstairs Heat Coming on with Downstairs


9 Feb 2021
Reaction score
United Kingdom
Hi All,

I have recently had a new smart thermostat fitted but since then I'm having an issue with my separate heating zones.

When the upstairs thermostat calls for heat, only the upstairs radiators come on as they should.

But when the downstairs thermostat calls for heat, both the downstairs and the upstairs radiators come on. The one note I will add to this though is that, when this happens, the upstairs radiators don't seem to get as hot as they would as if they were turned on properly by the upstairs thermostat... they just get very, very warm. It's a little odd and I am not sure of the relevance of this.

I have a system boiler with three zone valves: one for upstairs heat, one for downstairs heat and one for the hot water tank. All of the wiring runs through a central wiring box which obviously links the boiler with the relevant thermostats and zone valves. The electrician has returned and confirmed that the issue isn't with the wiring.

Part of me wonders if the upstairs zone valve is faulty but then, that would be a coincidence since the issue only seemed to start after the installation of the smart thermostats.

Any thoughts would be appreciated.

Sponsored Links
Sorry to bump this... but any thoughts anyone?
switch the downstairs zone only on, then feel the outlet side of the upstairs zone valve and see if it is letting heat through
Not that many Smart thermostats are designed for multi-zones, Drayton do a three zone and so does EPH in fact think EPH goes to 10 zones, but in the main CH and DHW as zones, as with programmable TRV heads there is really no need to use old fashion zone valves today, and the systems that use them don't tend to work with programmable TRV heads as well, a little thought and you will see no point, they are both doing the same job.

OK exception to every rule this home has two zone valves, one for annex and one for main house, in the main as annex is only used for storage, so nearly always turned off.

But first question has to be if modulating boiler or not, with a old oil boiler it is off or on, so having a valve turn off or on makes sense, with modern gas it turns up/down so controls also should turn up/down, and the traditional zone valve is on/off, so not fit for purpose, to heat a room to start with better if not starting from 6°C that's too cold, so you want in general two settings shall we call them eco and comfort, the eco holds the room at say 16°C and comfort at say 21°C and so not starting from scratch to heat room, so it can heat room in a reasonable time.

Also each room likely has a different schedule.

So the boiler will only switch off when every room is satisfied or if it can't turn down any lower, typical boiler 6 kW to 28 kW so once at 6 kW it has to start cycling, but the idea is the radiators are only as warm as required, the cooler the return water the more efficient the boiler is. Theory each TRV head sends a signal back to the wall thermostat and says if there is a demand for heat, in practice only a selected few report back, to reduce costs.
Sponsored Links
Thank you both for the responses.

Ianmcd, yep I had tried this. It's similar to the situation with the radiators... the outlet side for the upstairs zone is very warm when downstairs is on. But downstairs outlet side is scolding hot. The other way round, downstairs is stone cold when upstairs is on. Hot water is definitely getting through the upstairs zone valve when downstairs is on.

Ericmark, my boiler is a simple on/off. Tado (my smart thermostat) have confirmed that my setup would work - I checked with them before buying and have contacted them since. They now believe it is an electrical wiring issue - but I'm not convinced at all. As I say my electrician (who I trust) has even been back to double check.

I understand your logic with a more modern setup with individual TRVs. I have purchased 4 Tado ones and they work a treat. But even if I had all of my upstairs radiators off at their TRVs I would still prefer to not have the hot water flowing around the upstairs pipes if I can help it (especially because it has been plumbed to allow this).

Any further thoughts would be appreciated. Do I need to get a plumber out to check the zone valve? As I say... it's just too much of a coincidence that this would fail the day that I have a new smart thermostat system installed...
Most likely the zone valve passing a bit- i've got exactly the same problem with upstairs getting a bit warm when downstairs only is selected. Did wonder whether i was getting reverse circulation but been investigating today & flow is warmer than return.
Fortunately i have a spare in the bits box but think i'll leave it alone til this cold snap passes. Might be worth (as an experiment) removing the motor & gearbox (if its Honeywell 2 screws) and gently pliering the valve open and closed a few times.
Hi All.

@oldbutnotdead... did you ever confirm the cause of your issue?

No I didn't- Amazon had a good offer on Drayton Wiser kit at Easter, once I stuck my 10% discount on as well each WiFi TRV head cost about £15 so pulled the motorised valves out & put WiFi heads on all the rads.
A few things you could check before getting a plumber involved;
Compare the pipe temperatures before & after the upstairs zone valve with upstairs only on and with downstairs only on.
When downstairs only is on, check there is resistance when you operate the manual lever on the upstairs zone valve. For comparison, try the same operation with upstairs only on.
Other checks start involving a multimeter...good luck

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links